Etchants for removing titanium contaminant species from titanium substrates

ABSTRACT

Described are methods for removing titanium species, including titanium metal and titanium nitride, from titanium substrates. Hydrofluoric acid chemistries are traditionally disfavored for such tasks, as hydrofluoric acid tends to attack the underlying titanium substrate. A chemistry in accordance with one embodiment employs a mixture of hydrofluoric acid and hydrogen peroxide that vigorously attacks deposited titanium and titanium nitride without significantly attacking the machined titanium alloys normally used to form titanium components for semiconductor processing.

BACKGROUND

Semiconductor devices are built up using a number of material layers.Each layer is patterned to add or remove selected portions to formcircuit features that will eventually make up an integrated circuit.Some layers can be grown from another layer; for example, an insulatinglayer of silicon dioxide can be grown over a layer of silicon byoxidizing the silicon surface. Other layers are formed using depositiontechniques, typical ones being chemical vapor deposition (CVD),evaporation, and sputtering.

Deposition methods form layers using vaporized materials that condenseto form a film on the surface of interest. Unfortunately, the films thusformed are not limited to the surface of interest, but tend also to formon other surfaces within the reaction chamber. Thus, after substantialuse, a thick film of the deposited material accumulates on componentsand surfaces within the reaction chamber. These films eventually becometroublesome sources of contaminants. Etch processes also contaminateinside surfaces of reaction chambers, though by different mechanisms. Ineither case, the reaction chamber, including internal components, mustbe periodically cleaned or replaced.

Many process contaminants are removed using hazardous liquids.Unfortunately, the storage, use, and disposal of hazardous liquids andtheir vapors are dangerous and expensive, particularly when thesechemicals are used in large volumes. There is therefore a need forcleaning methods and systems that minimize the required amounts ofhazardous chemicals.

The difficulty and expense of dealing with hazardous chemicals are notthe only problems encountered when cleaning semiconductor processequipment. Some forms of contamination are so stubbornly attached to theunderlying material that removal of the contamination jeopardizes thepart to be cleaned. Each of FIGS. 1, 2, and 3 (prior art) illustrates anexemplary component and is used to describe a particular cleaningproblem addressed in the following disclosure.

FIG. 1 (prior art) depicts a stainless-steel shield 100 used to containtitanium-bearing vapors during physical vapor deposition (PVD) processesused to deposit layers of titanium and titanium alloys on semiconductorwafers. In confining such vapors, the interior surface 105 of shield 100becomes highly contaminated with layers of titanium and titaniumspecies, such as titanium nitride. Exterior surface 110 of shield 100also becomes contaminated, though to a lesser extent. Shield 100 musttherefore be periodically cleaned or replaced.

Conventional etchants that attack the titanium and titanium alloys alsoattack stainless steel. Immersing shield 100 in these etchants to removethe contaminants can therefore damage the underlying stainless steel.Exterior surface 110 is particularly vulnerable because that stainlesssteel lacks the thick contaminant layer of interior surface 105, and isthus exposed to etchants for a longer time. Pitting and roughening ofexterior surface 110 is undesirable for aesthetic purposes and becauserough surfaces trap undesirable contaminants when shield 100 is returnedto a process chamber. There is therefore a need for a method ofeffectively removing titanium contaminant species from shield 100without damaging the underlying stainless steel.

FIG. 2 (prior art) depicts an aluminum blocker plate 200 used todistribute gases evenly over a semiconductor surface. Blocker plate 200is used, for example, to evenly distribute silicon-bearing gases (e.g.silane) over the surface of a semiconductor wafer during silicondeposition processes. Blocker plate 200 includes a constellation ofsmall holes 205 through which pass the silicon-bearing gas. During suchdeposition processes, the surfaces of aluminum blocker plate 200,including the inner surfaces of holes 205, become contaminated withsilicon and silicon oxides. Blocker plate 200 must therefore beperiodically cleaned or replaced.

Oxides of silicon are difficult to remove from aluminum because commonsilicon etchants vigorously attack aluminum. A similar problem existsfor components of or layered with yttrium oxide or sprayed ceramic.Expensive components like blocker plate 200 are therefore discarded andreplaced rather than cleaned and reused. There is therefore a need inthe art for a way removing silicon and silicon-bearing contaminationfrom expensive aluminum, yttrium oxide, and sprayed ceramic parts.

FIG. 3 (prior art) depicts a diffusion tube employed in high-temperaturefurnaces to deposit polysilicon and silicon nitride on semiconductorwafers. Diffusion tube 300 can be of quartz or silicon carbide. Duringthe deposition of polysilicon or silicon nitride, these depositedmaterials built up on the inner surfaces of diffusion tube 300. After aperiod of use, the resulting contamination layers can begin to flakeoff, posing a serious threat of induced defects on the wafers beingprocessed. It is therefore necessary to periodically clean or replacediffusion tube 300.

Unfortunately, current methods of cleaning diffusion tubes areinadequate. In a typical process, one or more “spray balls” are insertedup into a vertically positioned diffusion tube 300. Etchants are thensprayed against the interior surfaces of diffusion tube 300 to dissolveaway the accumulated contamination layers. Spray balls do not applychemicals evenly; therefore contamination removal is slow and uneven.There is therefore a need of improved methods of restoring expensivediffusion tubes to a contamination-free state.

The examples of FIGS. 1-3 are in no way exhaustive of the problemsencountered as a result of contaminated semiconductor-processingequipment or of the types of parts that can be cleaned. Many otherexpensive components pose difficult cleaning problems. For example, sometitanium components become contaminated with titanium species, includingtitanium metal and titanium nitride. Known methods of removing titaniumspecies are labor intensive and potentially damage the underlyingtitanium substrate. There is therefore a need for methods of removingtitanium metal and titanium alloys from titanium substrates.

SUMMARY

The present invention is directed to methods, systems, and chemistriesfor cleaning various components of semiconductor process equipment.These components are of different types of materials and suffer fromdifferent types of contamination. The embodiments described hereinremove these contaminants using small amounts of chemicals and withminimal damage to the article being cleaned.

A method in accordance with one embodiment cleans articles withdifferently contaminated interior and exterior surfaces by using thosearticles to separate a cleaning vessel into separate chambers, onechamber for the interior surface and one for the exterior surface.Different chemistries are then applied to the differently contaminatedsurfaces. This embodiment reduces the required volume of etchant, andconsequently saves considerably on the purchase, handling, and disposalcosts associated with the use of toxic chemicals. One embodiment furtherreduces the requisite etchant volume using one or morevolume-displacement elements that displace some of the etchant volume.

A method in accordance with another embodiment removes layers ofstubborn silicon and silicon-nitride contamination from the interiorsurfaces of articles such as deposition tubes. In such embodiments, atube to be cleaned is gently rolled on it side while a portion of thetube's interior surface is exposed to an etchant. The tube is onlypartially filled with etchant to reduce the requisite etchant volume,and the rolling motion evenly exposes the contaminated surfaces to theetchant.

Another embodiment employs hydrofluoric acid to remove titanium species,including titanium metal and titanium nitride, from titanium substrates.Hydrofluoric acid chemistries are traditionally disfavored for suchtasks, as hydrofluoric acid tends to attack the underlying titaniumsubstrate. A chemistry in accordance with one embodiment includes amixture of hydrofluoric acid and hydrogen peroxide that vigorouslyattacks deposited titanium and titanium nitride without significantlyattacking the machined titanium alloys normally used to form titaniumcomponents for semiconductor processing. This chemistry can therefore beused to clean expensive titanium parts. In one embodiment, the chemistryfound to remove titanium and titanium-bearing contaminants from atitanium substrate without significantly attacking the substrate is amixture of less than about 2% hydrofluoric acid, from between about 6%and about 30% hydrogen peroxide, and the balance water. This mixture maybe selectively applied to contaminated areas.

In accordance with yet another embodiment, a hydrofluoric acid solutionis employed to remove silicon species from aluminum, yttrium oxide, andsprayed ceramic substrates. Silicon oxides are relatively inert, andconsequently resist most etchants. However, hydrofluoric acid has longbeen known to be effective at dissolving silicon oxide. Unfortunately,hydrofluoric acid strongly attacks aluminum, yttrium oxide, and sprayedceramic, and consequently damages expensive components. Methods used inaccordance with some embodiments remove silicon and silicon-bearingcontaminants from aluminum using a mixture of hydrofluoric acid andanhydrous acetic acid. The hydrofluoric-acid solution used in oneembodiment is 51% water and 49% hydrofluoric acid, so the etchantconsists primarily of water, hydrofluoric acid, and anhydrous aceticacid.

This summary does not limit the invention, which is instead defined bythe claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 (prior art) depicts a stainless-steel shield 100 used to containtitanium-bearing vapors during physical vapor deposition (PVD) processesused to deposit layers of titanium and titanium alloys on semiconductorwafers.

FIG. 2 (prior art) depicts an aluminum blocker plate 200 used todistribute gases evenly over a semiconductor surface.

FIG. 3 (prior art) depicts a diffusion tube employed in high-temperaturefurnaces to deposit polysilicon and silicon nitride on semiconductorwafers.

FIG. 4A depicts a volume-efficient cleaning system 400 that addressesthe problems of cleaning PVD shield 100 a FIG. 1.

FIG. 4B depicts a cleaning system 450, in which three shields 100 arestacked within containment vessel 405 of tank 400 for cleaning.

FIG. 5 depicts a cleaning system 500 in accordance with an embodimentthat cleans tubes like tube 300 described above in connection with FIG.3.

FIGS. 6A and 6B are respective side and front views of a cleaning system600 in accordance with an embodiment used to clean quartz and siliconcarbide tubes 300 of the type described above in connection with FIG. 3.

FIGS. 7A and 7B are respective side and front views of a cleaning system700 in accordance with another embodiment used to clean quartz andsilicon carbide tubes 300.

FIGS. 8A and 8B are respective side and from views of a cleaning system800 similar to system 600 of FIGS. 6A and 6B, like-numbered elementsbeing the same or similar.

FIG. 9 depicts a cleaning system 900 in accordance with an embodimentadapted to clean a wafer boat 905.

DETAILED DESCRIPTION

FIG. 4A depicts a volume-efficient cleaning system 400 that addressesthe problems of cleaning PVD shield 100 a FIG. 1. Tank 400 includes acontainment vessel 405, of volume displacement element 410, a firstinlet 415, and a second inlet 420. Inlets 415 and 420 can double asdrains, or separate inlets and drains can be provided. In thisillustrative example, tank 400 is employed to remove titaniumcontaminant layers from stainless steel shields of the type depicted inFIG. 1 and described above. The etchant used in the cleaning processincludes a mixture of hydrofluoric acid (HF) and hydrogen peroxide(H₂O₂). Specific etchant chemistries are detailed below.

Containment vessel 405 and volume displacement element 410 are made ofchemically resistant polypropylene, which is easy to clean and does notcontribute significant amounts of contamination to the cleaningsolution. Volume displacement element 410 is included to minimize therequired etchant volume. This aspect of tank 400 is important, as highlypure etchants, such as hydrofluoric acid and hydrogen peroxide, areexpensive. The expense of using hydrofluoric acid is exacerbated by theexpense of disposal and compliance with environmental regulations thatlimit the amount of fluorine injected into local sewage systems.

FIG. 4B depicts a cleaning system 450, in which three shields 100 arestacked within containment vessel 405 of tank 400 for cleaning. Each ofshields 100 includes first and second open ends, the first of which issmaller in diameter than the second. Shields 100 are stacked so that thefirst open ends from adjacent shields meet and the second open ends fromadjacent shields meet. The stacked shields 100 thus collectively form asomewhat cylindrical barrier separating an interior portion 455 ofvessel 405 from an exterior portion 460. Interior portion 455 andexterior portion 460 are simultaneously filled via respective inlets 420and 415, interior portion 455 with a titanium etchant and exteriorportion 460 with deionized water. Interior and exterior portions 455 and460 are filled until the levels of both portions reach the top of theuppermost shield 100. The etchant is then allowed to dissolve or weakenthe contamination on the interior surfaces of shields 100.

Filling interior and exterior portions 455 and 460 at the same rate tomaintain substantially equivalent fill levels in the interior andexternal portions maintains an equivalent pressure in those portions,and consequently prevents excessive mixing the etchant and water. Suchmixing can be further limited by providing a gasket material (not shown)between the bottom shield 100 and the bottom of vessel 405 and betweenadjacent shields 100. Such sealing is not generally necessary, as arelatively dilute acid solution formed in exterior portion 460 isdesirable to attack the relatively light titanium contamination on theexterior surfaces of shields 100. Some embodiments prevent excessiveetchant concentration in exterior portion 460 by circulating the waterin and out of portion 460 during the cleaning process. The etchantchemistry can also be adjusted during the cleaning process as desired.

As an added advantage, the jacket of water surrounding shields 100prevents shields 100 from heating excessively in response to theexothermic reaction normally used to remove stubborn titanium deposits.In other embodiments, the temperature of the cleaning process iscontrolled up or down using a heat exchanger, such as coils of stainlesssteel or polypropylene tubing, in one or both of exterior portion 460and interior portion 455. Such embodiments provide the additionaladvantage of displacing some percentage of either the cleaning solutionor the deionized water used in the respective interior and exteriorportions, and consequently reduce the amount of chemicals needed forcleaning and the amount of chemicals required for handling and disposal.Though not shown, a cover can be placed over 450 to collect and evacuatehazardous vapors and to protect operators from splashed chemicals.

Different types of cleaning solutions can be used depending on the typeof contamination and the item being cleaned. In this embodiment, inwhich a stainless-steel substrate is contaminated with species oftitanium, the use of a particular cleaning chemistry within interiorportion 455 has been shown to provide excellent contamination removalwhile minimizing the damage to the underlying stainless steel. Alsoimportant, the chemistry uses a relatively low concentration ofhydrofluoric acid, and consequently minimizes disposal costs andenvironmental impact. This chemistry includes about 60 partshydrogen-peroxide solution to one part hydrofluoric-acid solution. Thehydrogen-peroxide solution used in the chemistries discussed herein ispurchased as a ratio of 30% hydrogen peroxide to 70% water, and thehydrofluoric-acid solution is purchased as a mixture of 49% hydrofluoricacid and 51% water. The 60 to 1 mixture is therefore about 29.5%hydrogen peroxide, less than 1% hydrofluoric acid, and the balancewater. This mixture may include up to about 5% hydrofluoric acid andfrom about 6-30% hydrogen peroxide. To minimize the costs of chemicalsand their disposal, the preferred ranges of hydrofluoric acid andhydrogen peroxide for removing titanium from stainless steel are from1-2% hydrofluoric acid and from 10-29% hydrogen peroxide.

Though the articles being cleaned in FIG. 4B are stainless steel,hydrofluoric acid chemistries in accordance with some embodiments areused to remove titanium species, including titanium metal and titaniumnitride, from titanium substrates. Conventional methods of removingtitanium and titanium nitride employ a combination of chemical andmechanical processes that are collectively very labor intensive.Chemical processes include the use of hydrofluoric acid, but thesechemistries are disfavored for use in decontaminating titaniumsubstrates because the chemistries attack the substrates. As aconsequence of these shortcomings, very expensive titanium componentsare routinely discarded.

Applicants discovered an etchant chemistry that removes depositedtitanium and titanium nitride from titanium substrates without damagingthose substrates. This important chemistry employs a mixture ofhydrofluoric acid and hydrogen peroxide that vigorously attacksdeposited titanium and titanium nitride without significantly attackingthe machined titanium alloys normally used to form titanium componentsfor semiconductor processing. Applicants speculate that the etchantattacks deposited titanium and titanium nitride much more aggressivelythan the underlying titanium alloy due to compositional differences,physical differences, or both.

An etchant found to remove titanium and titanium-bearing contaminantsfrom a titanium substrate without significantly attacking the substrateis a mixture of less than about 2% hydrofluoric acid, from between about6% and about 30% hydrogen peroxide, and the balance water. This mixturemay be selectively applied to contaminated areas.

FIG. 5 depicts a cleaning system 500 in accordance with an embodimentthat cleans tubes like tube 300 described above in connection with FIG.3. System 500 operates using principles similar to those discussed abovein connection with FIGS. 4A and 4B to expose contaminated surfaces toetchants while protecting other surfaces from the etchants, reducing therequired volume of etchants, and controlling reaction temperature.

System 500 includes a containment vessel 505 removably attached to abase 510. An O-ring seal 515 prevents liquid from leaking from vessel505. A volume-displacement element 520 and optional gasket 525 are fixedto base 510. Before cleaning and cooling solutions are added to vessel505, vessel 505 is removed from base 510 and tube 300 is placed overdisplacement element 520. Tubes 300 can be heavy and expensive, so someversions of system 500 can be tilted to allow horizontal insertion oftube 300 over displacement element 520. Once system 500 is assembled asshown in FIG. 5, deionized water or some dilute cleaning solution isinjected into the region surrounding tube 300 via an inlet 530. Anetchant is also injected between the interior surface of tube 300 andvolume-displacement element 520 via a second inlet 535. A vent 527 andtube 540 allow air to escape as liquid is injected into containmentvessel 505 and tube 300. One or both of vent 527 and tube 540 can bevented for proper disposal of reaction gases.

Gasket 525 prevents the interior and exterior solutions from mixing. Agood seal is not necessarily important, however, as maintaining asimilar fluid depth inside and outside of tube 300 prevents significantmixing of the interior and exterior fluids. In some embodiments, thefluid level on the outside of tube 300 is maintained somewhat higherthan the fluid level inside tube 300 to prevent the etchant from flowingout of tube 300.

As noted above in connection with FIG. 3, typical contaminants on theinside of tube 300 include polysilicon and silicon nitride. Thesechemicals are conventionally removed using known hydrofluoric acid andnitric acid mixtures. The outside of tube 300 is exposed to deionizedwater in one embodiment, but other solutions might also be used. Inembodiments in which tube 300 is of a different form, of a differentmaterial, or contaminated with different materials on differentsurfaces, the chemistries of the interior and exterior solutions can bechanged accordingly.

FIGS. 6A and 6B are respective side and front views of a cleaning system600 in accordance with an embodiment used to clean quartz and siliconcarbide tubes 300 of the type described above in connection with FIG. 3.System 600 is adapted to evenly clean the interior surface of tube 300to remove polysilicon and silicon nitride contamination layers usingsmall volumes of etchants.

System 600 includes a storage vessel 605, typically of a chemicallyresistant polypropylene. Dams 610 are included at the bottom of vessel605 to displace some of the etchant. The shape and configuration of damscan be modified as appropriate to optimize the level and amount ofetchant and to accommodate different article shapes.

Tube 300 rests on two sets of rollers 615 disposed on a correspondingpair of axles 620. One of axles 620 includes a pulley 625 at one end,and pulley 625 is connected to a corresponding drive pulley 630 via abelt 635. In the depicted embodiment, belt 635 is exposed to anacid-based etchant 640 and is therefore made of an acid-resistantmaterial. In one embodiment belt 635 chemically resistant O-ringavailable from Dupont under the trademark KALREZ or from Greene TweedProducts under the trademark CHEMRAZ.

A driveshaft 642 connected to a motor (not shown) within a motor housing645 turns pulley 630 and consequently pulley 635, axles 620, and rollers615. Two sets of journals 650 support axles 620 and are mounted, in thisembodiment, to dams 610. The various components of system 600 are madeof chemically resistant materials, such as polypropylene or TEFLON.Journals 650, one of which is detailed in FIG. 6B, include two partsjoined using chemically inert bolts 655. Finally, a perforated drainpipe660 near the bottom of vessel 605 or some other form of drain isprovided to facilitate removal of etchant 640. Though not shown, vessel605 can include a lid, and the lid may be vented in a manner thatcollects potentially harmful vapors.

During operation, tube 300 is rotated slowly to evenly expose theinterior surface of tube 300 to etchant 640. An appropriate etchant forremoving polysilicon and silicon nitride includes equal volumes ofhydrofluoric and nitric acid solutions. Commercially availablehydrofluoric and nitric acid solutions include a significant percentageof water.

FIGS. 7A and 7B are respective side and front views of a cleaning system700 in accordance with another embodiment used to clean quartz andsilicon carbide tubes 300. System 700 is similar to system 600 of FIGS.6A and 6B, like-numbered element being the same or similar. In system700, dams 610 are eliminated; instead, a semi-cylindrical vessel 715reduces the requisite etchant volume.

In the embodiment of FIGS. 6A and 6B, the rollers, bearings, and beltare exposed to the cleaning solution. In system 700, a single axle 705supports tube 300 in a manner that isolates the drive mechanism frometchant 640. Tube 300 can be heavy, and axle 705 should be sufficientlyrigid to support tube 300. Though not shown, axle supports extendingfrom housing 645 into tube 300 can be added if necessary. Rollers 710 ofe.g. TEFLON can also be added to isolate axle 705 from the wet innersurface of tube 300. In other embodiments, rollers 710 are replaced witha chemically inert sleeve that covers a rigid (e.g., stainless steel)axle 705. Though optional, the two end rollers 710 are mildly cam-shapedto move etchant along the length of tube 300. Further, stops can beadded to prevent tube 300 from wandering too far on the rollers.

FIGS. 8A and 8B are respective side and from views of a cleaning system800 similar to system 600 of FIGS. 6A and 6B, like-numbered elementsbeing the same or similar. A cover 805 attached to the bottom end oftube 300 contains etchant 810 so the outer surface of tube 300 is notexposed to etchants. Cover 805 can be of polypropylene and may include achemically resistance O-ring (not shown) to prevent leakage. Chemicallyresistant fasteners 812 hold cover in place. A vent 815 may be used tofill and drain tube 300, and additionally serves as a vent for allowingreaction gases to escape. In the depicted embodiment, cover 805 istapered on the side facing into tube 300 from the outside diametertoward vent 815 to facilitate draining when tube 300 is placed upright.The outer vessel is protected from exposure to etchant, but still servesas secondary containment for spills and can contain e.g. water or arelatively mild cleaning solution to control process temperature andclean the outer surface of tube 300.

As noted above in connection with the discussion of FIG. 2, manyexpensive semiconductor-processing components are aluminum contaminatedwith silicon or silicon-bearing compounds. Eventually, thiscontamination will either impede gas flow or flake off, potentiallydamaging expensive semiconductor die.

Oxides of silicon are relatively inert, and consequently resist mostetchants. However, hydrofluoric acid has long been known to be effectiveat dissolving silicon oxides. Unfortunately, hydrofluoric acid stronglyattacks aluminum, anodized aluminum, yttrium oxide, and sprayed ceramic.Hydrofluoric acid thus damages surfaces made from these materials. Somecomponents can be cleaned using mechanical methods that scrape or blastaway exterior silicon contamination, but such methods do not work wellfor hard-to-reach surfaces, such as the interior surfaces of holes 205in blocker plate 200 of FIG. 2. Such methods may also remove or damagesensitive coatings, and are consequently difficult to apply tocomponents coated with yttrium oxide or sprayed ceramics.

Methods used in accordance with some embodiments to remove silicon andsilicon-bearing contaminants from aluminum using a hydrofluoric-acidsolution prepared by combining hydrofluoric acid with an anhydrous acid.The hydrofluoric acid used in one embodiment is 51% water and 49%hydrofluoric acid, so the etchant consists primarily of water,hydrofluoric acid, and an anhydrous acid.

A possible explanation for the effectiveness of this solution atremoving silicon oxides from aluminum substrates without damaging thealuminum is that the high concentration of anhydrous acid deprives theetchant of sufficient free water to attack the aluminum. Whatever themechanism, this chemistry has been found to remove silicon oxides fromaluminum, anodized aluminum, yttrium oxide, and sprayed ceramic surfaceswithout significantly damaging the underlying material. In one example,an aluminum blocker plate 200 with 0.025 to 0.030 inch diameter holescontaminated with silicon oxide was restored without any significantreduction in hole diameter.

In one embodiment, the hydrofluoric acid solution for removing siliconfrom aluminum contains between 0.5% and 30% hydrofluoric acid andbetween 50% and 99% anhydrous acetic acid. If the only water added tothe chemistry is provided with the commercially available hydrofluoricacid, then the etchant will typically contain less than about 25% water.A hydrofluoric acid solution is saturated with anhydrous citric acid inanother embodiment. Citric acid crystals may, for example, be added.

FIG. 9 depicts a cleaning system 900 in accordance with an embodimentadapted to clean a wafer boat 905, typically of quartz or siliconnitride. The chemistries employed are conventional, but a displacementelement 910 is added to a container 915 to displace a significantpercentage of the liquid used in cleaning. In one embodiment, forexample, displacement element 910 displaces over thirty gallons,reducing the liquid required to cover wafer boat 905 from over fortygallons to about seven gallons.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, while the contaminatedarticles are round in the forgoing examples, differently shaped partsand correspondingly shaped vessels and volume-displacement elementsmight also be used; and the methods described in connection with FIGS.5-8B can be applied to other types of cylindrical and substantiallycylindrical parts, including e.g. cylindrical shields conventionallyused inside deposition tubes. Therefore, the spirit and scope of theappended claims should not be limited to the foregoing description.

1. A method for removing a titanium-bearing contamination layer from acontaminated titanium substrate, the method comprising exposing thecontaminated titanium substrate to hydrofluoric-acid.
 2. The method ofclaim 1, wherein the hydrofluoric-acid is in a solution with hydrogenperoxide.
 3. The method of claim 2, wherein the solution includes lessthan two percent hydrofluoric acid and between six and thirty percenthydrogen peroxide.
 4. The method of claim 3, wherein the solutionincludes between seventy and ninety percent water.
 5. The method ofclaim 1, wherein the titanium-bearing contamination layer comprisetitanium nitride.
 6. The method of claim 1, wherein the titaniumsubstrate comprises a titanium allow.
 7. The method of claim 1, whereinthe titanium substrate is a semiconductor-process component contaminatedvia a semiconductor process.
 8. A method for removing a titanium-bearingcontamination layer from a substrate, the method comprising immersingthe contaminated substrate in a hydrofluoric-acid solution, thehydrofluoric-acid solution including less than five percent hydrofluoricacid and between six and twenty-nine percent hydrogen peroxide.
 9. Themethod of claim 8, wherein the solution includes more than sixty-fivepercent water.
 10. The method of claim 8, wherein the substrate isstainless steel.
 11. The method of claim 10, wherein the hydrofluoricacid solution contains less than two percent hydrofluoric acid.
 12. Themethod of claim 11, wherein the hydrofluoric acid solution includesbetween ten and twenty-nine percent hydrogen peroxide.
 13. A cleaningsolution comprising: a. hydrofluoric acid in a concentration of lessthan five percent; b. between six and twenty-nine percent hydrogenperoxide; and c. water.
 14. The cleaning solution of claim 13, whereinthe hydrofluoric acid concentration is less than two percent.
 15. Thecleaning solution of claim 13, in liquid form.
 16. The cleaning solutionof claim 13 having immersed therein a contaminated substrate oftitanium.
 17. The cleaning solution of claim 16, wherein the substratecomprises a titanium alloy.
 18. The cleaning solution of claim 16,wherein the contaminated substrate includes a contamination layercomprised of nitrogen.